Well, you might have a certain common misunderstanding of what a theory is. Virtually everything that is taken as fact is actually a theory because human perception of the universe is limited and incapable of developing a truly comprehensive understanding thereof. And theories are theories because they have evidential merit; they’re ideas that are capable of functioning as explanations for tangible observations. The fact that an idea is “just a theory” is not grounds to discount the value of that idea.

Germ theory is effective because it does well to explain why people get sick. As for COVID specifically, I haven’t personally observed the COVID virus infecting someone, but you haven’t personally observed someone getting COVID without being infected by the virus, so you have no grounds for claiming that the virus doesn’t exist.

What kind of toxin would you think could spread like a virus could? It would have to replicate within a living organism, and it would have to contend with a person’s immune system, i.e. basically what a virus does.